Hey since I seem to be shoot for the moon here (I was happy several posts back when I managed to get a ! in the column), I would love a new Column Type call it Symbols. You could put a $ (for a book to buy), a little calendar page (that could be used for books coming soon), a fancy N (for new books that haven't had metadata update yet), of course my ! and the check mark and x. People could use them for what ever they wanted. I'm sure others would have a few ideas for symbols. It is something very visual for people like me.

I have an idea that might work out here. It would also satisfy some long-running enhancement requests.

I think I can make calibre use a custom icon image for specific values in the column, probably by using the column's coloring specification. If the color is "something.png" (but without the quotes) instead of a color name, then when calibre sees the corresponding value it would use the image instead of the value.

You would put the file "something.png" into yourCalibreConfigFolder/resources/images, the same place that custom toolbar icons go. If calibre cannot find the image then it would display the text.

These custom icons would be displayed only on calibre's spreadsheet. They would not be displayed in the content server, book details, or other places that display values.